To determine if serotonin, histamine and other vasoactive amines play a role in the delayed cellular phase of inflammation. These chemicals were tested for their ability to induce the production of a monocyte chemotactic factor from human peripheral blood leukocytes. We have demonstrated that serotonin stimulates human leukocytes to produce a monocyte chemotactic factor. Peak production is at a 10 to the minus 4th power M to 10 to the minus 3rd power M concentration of serotonin and occurs at 48 hr. Histamine was found to synergistically interact with serotonin in the production of this factor. These studies suggest that serotonin and histamine may contribute to the induction of monocyte exudation in inflammation.